驅動超音波換能器之E類功率放大器及變壓器設計

Abstract

壓電效應於1880年由居禮兄弟(Pierre Curie and Jacques Curie)所發現，而超音波自十九世紀末期發展以來，已有多年的發展歷史。時至今日，超音波技術不論是超音波診斷儀、超音波清洗機、超音波測距儀皆已深入在我們的生活中。超音波可以傳遞於空氣、固體或液體等介質中，在應用上，超音波皆須使用換能器發射超音波。其中，介質為空氣之超音波，其換能器可分為開放式及封閉式換能器。開放式換能器能直接接觸空氣，其發射效率也較高，但有時因為必須防水、防塵或其他防護，換能器必須採封閉式的被保護於外殼中，造成能量傳遞的犧牲。 一般而言，在超音波發射系統中，最重要的就是功率放大器。應用於超音波之功率放大器大多為D類功率放大器，而D類功率放大器之輸出為方波。然而E類功率放大器之輸出為弦波，理論上在負載端擁有更好的效率及功率。因此，本論文以E類功率放大器進行設計，在固定直流電壓供應下，加入變壓器以驅動工作頻率為40kHz之封閉式空氣超音波換能器。本研究將先針對封閉式空氣超音波換能器，設計、模擬並實作E類功率放大器，而由於E類功率放大器之輸出功率由直流電源供應及負載大小決定。因此，在固定直流電源供應下，在換能器前端加入自行設計的變壓器後重新設計E類功率放大器，最後量測換能器在此電路驅動下之發射聲源聲壓。

Piezoelectric effect was discovered in 1880 by Pierre Curie and Jacques Curie, and there had been many years of history since ultrasound began to develop in the 19th century. Up to this day, ultrasonic technique such as Ultrasound Identification (USID), ultrasonic cleaning and ultrasonic range finding are all in every aspect of our lives. Ultrasonic waves can be transmitted to air, solid and liquid media. In application, ultrasonic waves are transmitted by transducers. Among all, transducers that transmit ultrasonic waves in air can be divided into two types: open and closed. The open transducer can be exposed to air directly and has higher transmission efficiency. However, because of waterproofing, dustproofing and other protection, the transducer must be enclosed and protected in a shell, which causes loss of energy transmission. Generally speaking, the most important thing in the ultrasonic transmitting system is the power amplifier. Most ultrasonic power amplifiers are Class D PAs and its output is square wave signal. However, the output of Class E PAs is sine wave signal, which means Class E PAs have better latter efficiency and power theoretically. Thus, this article will design a Class E PA and a transformer with fixed DC power supply to drive a 40kHz closed-type ultrasonic transducer. This research will perform design, simulation and actual production of Class E PAs focused on closed-type ultrasonic transducer. Moreover, due to the output power of Class E PAs depend on the DC power supply and the load, a transformer is added and the Class E PA is redesigned in case of the DC power supply is fixed. Finally, the acoustic pressure level of the transducer driven by the designed circuit is measured to establish a design process of transducer driver circuit.